1. Field of the Invention
The invention deals with the field of light devices for motor vehicles, notably light devices which use a plurality of light-emitting diodes, LED, to produce at least one light function of a motor vehicle.
2. Description of the Related Art
A light-emitting diode, LED, is a semiconductor electronic component that emits light when it is passed through by an electrical current of a specific intensity. A property which characterizes an LED is its forward voltage Vf. That is the voltage drop measured at the terminals of the LED when the latter is passed through by an electrical current and emits light. The development of increasingly efficient semiconductor components is resulting in the emergence of LEDs that have increasingly lower forward voltages. The forward voltage of an LED is, at an equal current, a decreasing function of its semiconductive junction temperature.
In the motor vehicle industry, and in particular in the field of light devices for motor vehicles, the use of LEDs is increasingly recommended to replace the incandescent light sources traditionally used. This is because the low electrical consumption of LEDs has an undeniable advantage. In addition, a plurality of LEDs can be placed on a predetermined line, thus making it possible to create interesting and individual optical signatures of the lights of a motor vehicle. It is known in practice to use a plurality of LEDs connected in series to produce a light function of a motor vehicle such as, for example, the daytime running lights, direction indicator or high beam function. When the junction of one of the LEDs of such a series assembly is defective, the LED concerned is said to be short-circuited. Headlights of a motor vehicle can be subject to widely varying meteorological conditions. Thus, LEDs forming part of such a headlight need to be able to operate at very low temperatures of the order of −20° C. or less, and at operating temperatures of the device which can exceed 80° C.
It is known practice to detect a short-circuited LED by comparing the voltage at the terminals of the series assembly to an aggregate forward voltage. Such a known solution starts from the principle that if the measured voltage is below N·Vf, N being the number of LEDs connected in series and Vf being their forward voltage, then one of the LEDs is short-circuited. With an increasing number of LEDs mounted in series, this method, may, however, in many instances, produce false positive detections. For example, over a temperature range ranging from −40° C. to 90° C., the forward voltage Vf of an LED can exhibit variations of approximately 0.6 V. The minimum forward voltage Vfmin, at maximum temperature, of the same LED can be equal to approximately 2.3 V. In such a configuration, the voltage at the terminals of a series assembly comprising N LEDs can exhibit variations of N·0.6 V due exclusively to the junction temperature of the LEDs. Clearly, starting from N=6, the amplitude of these variations far exceeds the minimum forward voltage of each of the LEDs of the assembly. Consequently, a short-circuit of an LED may no longer be detected with certainty and the known method is likely to produce false alarms.
The patent document DE 10 2007 024 784 B4 describes a device capable of detecting the short-circuit of an LED in a series assembly. When a failure is detected, an alert signal is notified to the user of the vehicle through the internal information system of the vehicle. The solution described is not capable of taking into consideration the forward voltage variations linked to the junction temperature of the LEDs.
The patent document U.S. Pat. No. 7,638,947 B2 presents a device intended to detect a short-circuit of an LED in a series assembly. According to one embodiment described, the device can be adapted to take into consideration a variation of the forward voltage as a function of the temperature of the LEDs of the series assembly. Nevertheless, the proposed solution requires the presence of dedicated electronic components on the printed circuit board which supports the LEDs. The dedicated components have to be arranged in a specific manner at the terminals of at least one of the LEDs, which generates an increased production cost, an additional constraint in the design of such a printed circuit board, and the potential loss of space on the printed circuit board.